1. Field of the Invention
The present invention relates to circular dichroism spectrometers, and more specifically, to alignment of optical positions of elements in a circular dichroism spectrometer.
2. Description of the Related Art
Two given molecules that consist of identical atoms and are expressed by an identical chemical formula may have different stereostructures, like mirror images. This relationship is called chirality, and the stereostructures have left-handed (labeled L) and right-handed (labeled D) forms. The L and D forms have the same molecular weight and the same binding energy, and their physical properties, such as concentration, melting point, boiling point, refractive index, and thermal conductivity, are also the same. It is difficult to distinguish the stereostructures.
The L and D forms, however, may have totally different biochemical properties, and, from a pharmaceutical or biochemical perspective, it is necessary to determine the absolute configuration of the molecules. The stereostructures are analyzed based on optical rotation, which is a property many chiral molecules generally have.
When the optical rotation is analyzed, right-handed circularly polarized light and left-handed circularly polarized light are radiated onto a sample, and the difference in absorbance is measured. This difference in absorbance is very small in many cases and increases just around the absorption peak of the sample. Therefore, the optical rotation of a sample is generally analyzed around the absorption peak of the sample.
Circular dichroism (CD) spectrometers that measure ultraviolet-visible CD spectra predominated in the past. After development of the technology, vibrational circular dichroism (VCD) spectrometers that measure infrared VCD spectra have been becoming more widespread. In comparison with CD spectra, VCD spectra allow more absorbers to be observed in accordance with the transitions of molecular vibrational states and offer more information.
The method of measuring VCD spectra will now be described. L-form absorbance AL is measured; D-form absorbance AR is measured; and the difference ΔA (ΔA=AL−AR) in absorbance is calculated. Then, the ellipticity θ is obtained as given below:tan θ=ΔA/(AL+AR)
Next, molecular extinction coefficients (εR, εL) are obtained as given below:θ·180/π·10·M/c=3300(εL−εR)where M stands for a molecular weight, and c stands for concentration.
When εL−εR is positive, the elliptically polarized light is right-handed; when εL−εR is negative, the elliptically polarized light is left-handed.
In the X-Y coordinate system with the vertical axis representing ΔA or εL−εR of the L form and the horizontal axis representing the wavelength λ, a circular dichroism spectrum (CD spectrum) can be plotted. The CD spectra of the L and D forms plotted on the same graph will have symmetrical shapes and different absolute values.
The VCD spectrometer uses a photoelastic modulator (PEM). The PEM includes the piezoelectric element attached to a ZnSe crystal. The ZnSe crystal is distorted by the piezoelectric element to which a voltage applied. Accordingly, birefringence occurs in the ZnSe crystal. When the applied voltage varies with a predetermined frequency, left-handed circularly polarized light and right-handed circularly polarized light can be generated alternately.